Wait until they've finished flowering, then cut back to the last two or three pairs of green leaves on each stem - they should then produce new shoots from low down. A bit of a feed at the same time won't go amiss.

I've got a very tall woody one that I'm fond of as it was one of the first plants that I grew from seed last year. It is flowering now so when it finishes I'm going to follow Alina W advice & see what happens.

I do agree with tdc, they do become very straggly. The upside is that you can easily collect seeds & grow some more for next year.

I've also got the perennial types & will be trying to take some cuttings off these as from what I have been told they take really easily

Thanks very much for all the replies, I will try out all of the advice, ie cutting back as Alina suggests, but as mine are Erysimum I will also take cuttings and collect the seeds, all advice much appreciated, cheers